1. Field of the Invention
The present invention relates to a method for forming rustproof film with a synthetic resin powder coating material on a core wire and surrounding wires of a PC strand used as tensioning member or stay cable for post-tensioning or pre-tensioning in prestressed concrete used for structures such as architectural constructions and civil engineering structures, or of a PC strands used as stay member or stay cable for marine structures and cable-stayed bridges susceptible to salt corrosion, and a PC strand obtained by this method.
2. Prior Art
A PC strand generally has a twisted structure of plural surrounding wires twisted around a core wire. The reason for using such a structure is to impart flexibility to the PC strand, and to form helical grooves with the twisted surrounding wires and thus provide a sufficient shear resistance for wires embedded in concrete. Accordingly, there is a need for a rustproof processing method for the PC strand that does not interfere with these characteristics. Currently, a number of rustproof processing methods for the PC strand are known.
One example of such known conventional techniques is “Rustproof film forming and processing method for PC strand” disclosed in Japanese Patent 2691113. In this technique, twisted portions of a PC strand are temporarily untwisted in sequence. The untwisted portions are maintained with a spread maintaining means, and the excess core wire is adjusted. A deposition coating of synthetic resin powder coating material is then formed on whole outer peripheral surfaces of the untwisted core wire and surrounding wires. The deposition coatings are then heat-fused to form films on the whole outer peripheral surfaces of the core wire and surrounding wires. The core wire and the surrounding wires are then twisted again alter cooling the films.
Since the PC strand formed in this manner is formed with coats of 200 μm on the core wire and the surrounding wires by forming an adhesion film by spraying a powder coating material on the entire outer peripheral surfaces of the core wire and the surrounding wires respectively, and heating the adhesion film at a temperature of 250° C., characteristics such as flexibility required as the PC strand and shearing resistance with respect to concrete are not impaired at all and, in addition, the rustproofing function is also sufficient, so that this rustproofing method of this publication is thus praised as the ultimate rustproofing method of a PC strand.
Another known conventional technique is “Method of forming rustproof film on PC strand” disclosed in Japanese Patent No. 4676009. The method is performed in a series of line including: untwisting the PC strand and loosening surrounding wires from the core wire; forming a resin coat by applying a synthetic resin powder coating material on outer peripheral surfaces of the core wire and the surrounding wires in the loosened state respectively; heating the same to adhere each other uniformly and cooling the same; and subsequently twisting the surrounding wires around the core wire into an original state, wherein the heating includes a pre-heating before applying the synthetic resin powder coating material and a post-heating alter the application of the synthetic resin powder coating material, the temperature of the pre-heating is set within a range from 150 to 250° C. and the temperature of the post-heating is set within a range from 120 to 220° C., the temperature of the pre-heating is set to be 30 to 130° C. higher than the post-heating, the synthetic resin powder coating material having an average particle diameter of 40 to 50 μm is used in order to form the resin film having a predetermined thickness, and the speed of the line is set to 5 to 10 m/min.
In this manner, the temperature of the pre-heating is set to be higher than that of the post-heating, the synthetic resin powder coating material has an average particle diameter of 40 to 50 μm with that having particle diameters from 10 to 100 μm distributed substantially uniformly, and the line speed is set to 5 to 10 m/min, whereby the productivity is improved, cost reduction is achieved, and efficient formation of a uniform and preferable coat without impairing flexibility and a shearing resistance with respect to concrete is achieved.
In the first conventional technique described above, the thickness of the rustproof resin film is 200±50 μm, and the method in this technology is evaluated to be an ultimate rustproofing method. However, in order to form the resin film having the thickness described above, the line speed needs to be 4.5 m/min at maximum and, if the line speed is faster, than 4.5 m/min, a designed film thickness cannot be obtained, and hence a problem of low production efficiency exists.
In the second conventional technique, the temperature of the pre-heating needs to be set to be 30 to 130° C. higher than the temperature of the post-heating, and the synthetic resin powder coating material needs to be a selected powder coating material having particle diameters of 10 to 100 μm distributed substantially uniformly and having an average particle diameter in a range from 40 to 50 μm to be used. In addition, if the temperature of the pre-heating is increased, the powder coating material coated on the wires are abruptly heated and melted, and hence a hardening reaction of the powder coating material makes progress and hence the viscosity of the coating resin is increased before air present in gaps among the particles of the powder coating material being in contact with a basis material of the core wire or the surrounding wires goes out completely. Therefore, the air present in the gaps is trapped between the basis material and the coating resin and hence a foaming phenomenon occurs. The foaming phenomenon may result in formation of pinholes in the resin coating. Simultaneously, not only the probability of increase of trapped air bubbles increases with increase in particle diameter of the powder coating material and, furthermore, the probability of the formation of the pinholes is increased, but also the coat alter the application tends to be uneven and the texture of surface tends to be deteriorated. Accordingly, the average particle diameter of the powder coating material needs to be controlled within a selected or carefully selected range from 40 to 50 μm. If the control of the selected or carefully selected range of particle diameter of the powder coating material is neglected, since the temperature of the pre-heating is high, the air in the gaps among the particles is trapped and hence the probability of formation of the pinholes is increased due to the abrupt heating. Simultaneously, if the powder coating material does not have the selected or carefully selected particle diameter, the coat having the designed film thickness cannot be formed and hence the cost of material is increased inevitably, which results in a problem of high manufacturing costs.
Particularly, in the first conventional technique, nature and performance of the coat formed by the pre-heating and the post-heating (formation of the pinholes), the relationship between the line speed and the resin powder and improvement of productivity, and efficient formation of preferable coats are not described at all.
In contrast to the first conventional technique, in the second conventional technique, the temperatures of the pre-heating and the post-heating, the particle diameter of the coating material, and the line speed are specified to improve the productivity and achieve the cost decrease, and efficient formation of a uniform and preferable coat is achieved without impairing the flexibility and the shearing resistance with respect to concrete. However, since the temperature of the pre-heating is high, even when the powder coating material ejected from the electrostatic gun via air conveyance comes into contact with and the outer peripheral surfaces of the untwisted and the loosened core wire and surrounding wires respectively and fuses rapidly in the coating step, the powder coating material having a portion fused partly and lowered in viscosity is pushed by an air conveyance pressure (wind pressure) without welding and is oversprayed (streamed), so that an extremely thin string (cob-webbing phenomenon) may be generated. Since the extremely thin string is light weighted, it floats and is tangled gradually in a coating booth and becomes like cotton fiber (like a cotton candy), is adhered to the coats on the core wire and the surrounding wires in the loosened state, and the surface of the coat is formed with projections or becomes rough, which leads to a shape failure.